This invention relates to small signal amplifiers, particularly biological signal source amplifiers of the differential type that recover rapidly after presentation of an overload condition.
Input amplifiers for apparatus intended for biological signal measurement applications are usually designed for operation with very low voltages and currents. As a consequence, such input amplifiers are easily overloaded if subjected to stray signals associated with muscle action, high resistance electrode connections, stray fields, therapeutic stimulating currents of various kinds, and the like. In addition, such amplifiers are typically of the differential type whose output signal reference level may be different from the input reference level of equipment they are called upon to operate, such as, for example, a strip chart recorder, thereby creating a problem of transmitting the amplifier output signal to a subsequent circuit without a dc offset.
As an example, in a cardiac monitoring and electrical pacing apparatus for use with persons suspected of suffering from life-threatening cardiac arrhythmia, the input circuit for detecting the very small signals associated with cardiac electrical activity (hereinafter referred to for convenience as electro-cardiac signals) also receive much larger level signals associated with non-cardiac muscular activity, which often occurs as a consequence of pacing or defibrillation. Moreover, the input circuit is typically connected to the same patient electrodes as are the electrical pacing or defibrillation pulse generators. Under such circumstances the amplifier can be overloaded for a substantial period (corresponding to several beats at the normal heart rate) during and following muscular activity, particularly that caused by the application of a pacing or defibrillating pulse, at a time when it is of critical importance to know whether or not the stimulating pulse has performed its function. At such times, even a few seconds waiting for the input amplifier to recover from overload conditions can delay accurate diagnosis and thus affect the health of the patient. Further, the output of the amplifier, which may be of the differential input type having an output referenced to a voltage level other than power supply ground, should be transmitted to the subsequent circuitry without the introduction of a dc offset or other level distortion in order to ensure that the full dynamic range of that subsequent circuitry may be utilized and that the transmitted signal is as faithful to the output signal as possible.
Other methods have been previously utilized to solve the problem of overloading a biological signal source amplifier, particularly in response to the application of a therapeutic stimulating pulse. For example, Diack et al., U.S. Pat. Re. No. 30,750, describes the use of diodes placed across the amplifier input to clip the input signal to a maximum level and the use of an amplifier that can be gated off during the application of a pacing pulse. However, the voltage across a forward-biased diode is at least an order of magnitude higher than the amplitude of the electro-cardiac signals to be measured, and gating off the amplifier may not entirely prevent high level signals from overloading subsequent circuits.
Another approach facilitating recovery from overload has been described in Lewyn et al., U.S. Pat. No. 4,114,627, wherein the input circuit is automatically disconnected from the input amplifier during the application of a therapeutic pulse, and for a period of time after the pulse has occurred the input circuit is clamped through a resistor to signal ground. However, this approach will not respond to unpredictable signals such as those resulting from muscular activity, and is limited in its ability to expedite recovery by the passive clamping to ground.
Other technical references that may be of general interest with respect to the invention described and claimed herein are: Gobeli U.S. Pat. No. 3,656,487; Welborn, et al. U.S. Pat. No. 3,716,059; Gobeli, et al. U.S. Pat. No. 3,927,677; Brownlee, et al. U.S. Pat. No. 3,949,759; Wickham U.S. Pat. No. 3,972,334; Rockland, et al. U.S. Pat. No. 3,088,140; Langer, et al. U.S. Pat. No. 4,184,493; Langer, et al. U.S. Pat. No. 4,202,340; and Nordling U.S. Pat. No. 4,261,365.
Therefore, it can be seen that there is a need for a better means of facilitating recovery of a biological signal source amplifier from an input overload, and for a means for accurately translating the output of such an amplifier to a different signal level.